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ChemPhysChem

Production of Ultracold 87Rb133Cs in the Absolute Ground State: Complete Characterisation of the Stimulated Raman Adiabatic Passage Transfer

Peter K. Molony, Philip D. Gregory, Avinash Kumar, C. Ruth Le Sueur, Jeremy M. Hutson, Simon L. Cornish

Published Jul 5, 2016• 37 citations
Publisher

Abstract

RbCs molecules in the electronic, rovibrational and hyperfine ground state, using stimulated Raman adiabatic passage to transfer the molecules from a weakly bound Feshbach state. We measure one-way transfer efficiencies of 92(1)% and fully characterise the strengths and linewidths of the transitions used. We model the transfer, including a Monte Carlo simulation of the laser noise, and find this matches well with both the transfer efficiency and our previous measurements of the laser linewidth and frequency stability.

DOI
10.1002/cphc.201600501

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arXiv:1409.1485•Physical Review Letters•Dec 17, 2014•460 citations
RbCs molecules in the electronic, rovibrational and hyperfine ground state, using stimulated Raman adiabatic passage to transfer the molecules from a weakly bound Feshbach state. We measure one-way transfer efficiencies of 92(1)% and fully characterise the strengths and linewidths of the transitions used. We model the transfer, including a Monte Carlo simulation of the laser noise, and find this matches well with both the transfer efficiency and our previous measurements of the laser linewidth and frequency stability.
This thesis reports the creation and trapping of 87RbCs molecules in the absolute ground state with a temperature of 1 uK. We build a tunable narrow-linewidth laser system at 1550 nm and 980 nm, using a single high-finesse optical cavity as a reference for both colours. We use fibre-coupled electro-optic modulators to continuously tune both lasers. These allow a novel measurement of the free spectral range of the cavity to better than 1 part in 10^6. We perform one- and two-photon spectroscopy on 87RbCs Feshbach molecules and identify a suitable intermediate state for transfer to the molecular ground state. We measure the electric dipole moment of the molecular ground state as 1.225(3)(8) D, and demonstrate the highest lab-frame dipole moment of any ultracold molecular system at the time of measurement. We transfer the molecules to the electronic, rovibrational and hyperfine ground state using stimulated Raman adiabatic passage, with 88% efficiency. We measure the transition strengths and excited state linewidth for this transfer route. We develop a model for the transfer which includes the effect of laser linewidth, and find excellent agreement with experimental data. The molecular sample is trapped in an optical dipole potential, and has a lifetime of 0.89(6) s. We reference the STIRAP lasers to a novel design of frequency comb which uses difference frequency generation to cancel the carrier-envelope offset. We use this to measure the binding energy of the molecules as h x 114 268 135.24(4)(3) MHz. To our knowledge, this is the most precise determination of the dissociation energy of a molecule to date. Finally, we report progress toward loading the molecules into a 1D optical lattice at 1064 nm. We develop the tools and methods to characterise a lattice, and demonstrate trapping of Feshbach molecules in both a 1D optical lattice and a harmonic optical potential at 1064 nm.
The thesis reports transferring the ultracold RbCs molecules to the absolute ground state and a trapping light that eliminates the differential AC Stark shift. We build a laser system to perform Stimulated Raman Adiabatic Passage (STIRAP) for the molecules trapped in tweezers, which transfer the molecules from the weakly- bounded state to the absolute ground state. We use the offset cavity stabilisation technique to stabilise the laser frequencies to a passive cavity to narrow the laser linewidths while keeping the laser frequency tunable. We then align the beams to the molecules. We demonstrate a “magic” laser frequency to eliminate the differential AC Stark shift of the rotational states in the ground vibrational state of RbCs molecule and to increase the rotational coherence time of multiple states. We study the AC Stark effect caused by the trapping light. We calculate the polarisabilities of the rotational states near a narrow electronic from X ^1Σ^+ state to b^3Π_0 state and found at certain laser frequency the polarisabilities become the same. We measure the transition frequencies to different vibrational states in the excited electronic state experimentally. We perform microwave spectroscopy to measure the light shift and show that the shift vanishes at the magic frequency. We confirmed the coherence in 1 ms timescale by measuring the Ramsey fringe contrast.
Many experimental platforms for quantum science depend on state control via laser fields. Frequently, however, the control fidelity is limited by optical phase noise. This is exacerbated in stabilized laser systems where high-frequency phase noise is an unavoidable consequence of feedback. Here we implement an optical feedforward technique to suppress laser phase noise in the stimulated Raman adiabatic passage state transfer of ultracold RbCs molecules, across 114 THz, from a weakly bound Feshbach state to the rovibrational ground state. By performing over 100 state transfers on single molecules, we measure a significantly enhanced transfer efficiency of 98.7(1)% limited only by available laser intensity.
We report the binding energy of 87Rb133Cs^{87}\mathrm{Rb}^{133}\mathrm{Cs}87Rb133Cs molecules in their rovibrational ground state measured using an offset-free optical frequency comb based on difference frequency generation technology. We create molecules in the absolute ground state using stimulated Raman adiabatic passage (STIRAP) with a transfer efficiency of 88%. By measuring the absolute frequencies of our STIRAP lasers, we find the energy-level difference from an initial weakly bound Feshbach state to the rovibrational ground state with a resolution of $\ensuremath{\sim}5$ kHz over an energy-level difference of more than 114THz114\phantom{\rule{0.28em}{0ex}}\mathrm{T}\mathrm{Hz}114THz; this lets us discern the hyperfine splitting of the ground state. Combined with theoretical models of the Feshbach-state binding energies and ground-state hyperfine structure, we determine a zero-field binding energy of $h\ifmmode\times\else\texttimes\fi{}114\phantom{\rule{0.16em}{0ex}}268\phantom{\rule{0.16em}{0ex}}135.24(4)(3)\phantom{\rule{0.28em}{0ex}}\mathrm{M}\mathrm{Hz}$. To our knowledge, this is the most accurate determination to date of the dissociation energy of a molecule.
One possibility for the creation of ultracold, high phase space density quantum gases of molecules in the rovibronic ground state relies on first associating weakly-bound molecules from quantum-degenerate atomic gases on a Feshbach resonance and then transferring the molecules via several steps of coherent two-photon stimulated Raman adiabatic passage (STIRAP) into the rovibronic ground state. Here, in ultracold samples of Cs2 Feshbach molecules produced out of ultracold samples of Cs atoms, we observe several optical transitions to deeply-bound rovibrational levels of the excited 0(u)+ molecular potentials with high resolution. At least one of these transitions, although rather weak, allows efficient STIRAP transfer into the deeply-bound vibrational level (see text for symbols)v = 73 > of the singlet X1 sigma(g)+ ground state potential, as recently demonstrated (J. G. Danzl, E. Haller, M. Gustavsson, M. J. Mark, R. Hart, N. Bouloufa, O. Dulieu, H. Ritsch, and H.-C. Nägerl, Science, 2008, 321, 1062). From this level, the rovibrational ground state (see text for symbols)v = 0, J = 0 > can be reached with one more transfer step. In total, our results show that coherent ground state transfer for Cs2 is possible using a maximum of two successive two-photon STIRAP processes or one single four-photon STIRAP process.
We report the creation of a sample of over 1000 ultracold 87Rb133Cs^{87}\mathrm{Rb}^{133}\mathrm{Cs}87Rb133Cs molecules in the lowest rovibrational ground state, from an atomic mixture of 87Rb^{87}\mathrm{Rb}87Rb and 133Cs^{133}\mathrm{Cs}133Cs, by magnetoassociation on an interspecies Feshbach resonance followed by stimulated Raman adiabatic passage (STIRAP). We measure the binding energy of the RbCs molecule to be $hc\ifmmode\times\else\texttimes\fi{}3811.576(1)\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ and the $|{v}^{\ensuremath{'}\ensuremath{'}}=0,{J}^{\ensuremath{'}\ensuremath{'}}=0⟩$ to $|{v}^{\ensuremath{'}\ensuremath{'}}=0,{J}^{\ensuremath{'}\ensuremath{'}}=2⟩$ splitting to be $h\ifmmode\times\else\texttimes\fi{}2940.09(6)\text{ }\text{ }\mathrm{MHz}$. Stark spectroscopy of the rovibrational ground state yields an electric dipole moment of 1.225(3)(8) D, where the values in parentheses are the statistical and systematic uncertainties, respectively. We can access a space-fixed dipole moment of 0.355(2)(4) D, which is substantially higher than in previous work.